A hydraulic circuit comprises two essential components that carry out the conversion of mechanical energy into hydraulic energy and vice-versa. These two components consist of a pump and a motor. In a hydraulic circuit, in addition to these two components, it is required for there to be devices that carry out a control action on the fluid. Due to operating requirements of the circuits it is thus necessary to intervene on the essential hydraulic magnitudes of the fluid, in other words pressure and flow rate, as well as on the direction of the fluid flow.
The devices proposed to perform this control function of the fluid are called valves, which can be grouped into three main categories:                directional valves;        flow rate valves;        pressure valves.        
In general, the above categories of valves can only take up two states: actuated and not actuated. This means that the state taken up by the actuator of the valve, typically consisting of a solenoid, can cause the closure and the complete opening of the valve shutter, but it does not allow any intermediate position of such a shutter.
Moreover there are continuous positioning valves, like proportional valves, which are able to take up precise intermediate positions between the completely open position and the totally closed position of the shutter. This way of operating makes it possible not only to control the direction of the flow, but also to adjust both the flow rate and possibly the pressure thereof downstream of the shutter through the load losses caused by the shutter itself.
In the panorama of valves available, a last important category of directional and/or proportional valves concerns two-stage valves. A two-stage valve substantially consists of two directional valves joined in a single valve body. The control system of the two-stage valve, typically consisting of a solenoid, acts on a small directional valve, or pilot valve, which uses the fluid of the hydraulic circuit to control the main valve. This type of two-stage valve becomes necessary in large sized valves, which operate in high pressure hydraulic circuits, because the great forces required by the direct actuation of the shutter could not be generated only by the solenoid. In two-stage valves, on the other hand, the solenoid provides sufficient force to actuate only the pilot stage, of small size, which through suitable pilot lines hydraulically controls the shutter of the main valve.
A typical scheme of a two-stage valve is shown in FIGS. 1 and 2. In FIG. 1 the pilot valve is indicated with reference numeral 100, whereas the main valve is indicated with reference numeral 102. The pilot valve 100 is provided with an actuator device 104, for example consisting of a solenoid. By energising the solenoid 104, the shutter 106 of the pilot valve 100 moves axially to the left with reference to FIG. 1. This allows the fluid, conveyed through suitable channels, to hydraulically actuate the shutter 108 of the main valve 102, axially moving it to the right with reference to FIG. 1.
A known two-stage valve, therefore, is particularly bulky and heavy, since it consists of two distinct valves interconnected through suitable channels. In addition, although it is provided with an actuator that controls only the shutter of the pilot valve, a known two-stage valve in any case consumes a substantial amount of electrical energy in order to be able to operate correctly.
A known two-stage valve, in particular a two-stage valve with on/off function, is described in document EP 2 239 486 A1. Another known two-stage valve is described in document US 2012/0151913 A1.